When light is passed through an optical fiber that guides a plurality of modes, the different modes will generally have different optical phase shifts associated with them that depend on the length of the fiber and the differences in phase velocities of the different modes. This will cause interference effects when light from the different modes is mixed. If light intensity is measured at a point on the end of the fiber or in any region removed from the end where the modes are mixed (and less than the entire output region), the observed intensity will depend on the relative phases of the modes, and will generally change if the phases change. A very small disturbance of the fiber can cause significant inter-modal phase shifts, so this principle can be used to construct a sensor for motion, vibration, pressure changes, or any other effect that can be coupled to a fiber to induce phase changes. This type of sensor is called a modal-domain sensor. Such modal domain sensors are known in the art. See e.g. Bucaro, Dardy and Carome, "Optical Fiber Acoustic Sensor", Applied Optics, Vol. 16, No. 7, July 1977, pp. 1761-62; Butter U.S. Pat. No. 4,287,684; Kingsley et al., "Multimode Optical Fibre Phase Modulators and Discriminators", Electronics Letters, Vol. 14, No. 11, May 25, 1978, pp. 322-324 and 335-337; and, Kul'chin et al., "Investigation of the Influence of an Aperture stop on the Signal/Noise Ratio in a Single-Fiber Interference-Type Sensor", Sov. J. Quantum Electron., Vol. 16, No. 8, Aug. 1986, pp. 1092-1095.
To detect a good signal from a modal-domain sensing fiber, it is necessary to have both mixing of the modes and restriction of the light that is received by the detector. Mixing is necessary to create interference. Restriction is necessary because for small disturbances of the fiber the total amount of power that emerges is constant, even though the spatial or angular distribution of the power may change because of mixing and phase changes.
In most implementations, mixing will occur without much effort because light from more than one mode strikes each region of the detector. In any implementation, explicit means are necessary to achieve good restriction. Some restriction will always occur naturally because the detector, and the optical path between it and the fiber, are never perfectly homogeneous; but such "natural" restriction cannot be relied upon to give high sensitivity or controlled performance.
Many applications of sensors require that the sensor and the electronics for analyzing the signals from it be remote from each other. Furthermore, it is necessary that disturbances of the means for coupling signals between the sensor and the electronics should not create effects that can be confused with signals from the sensor. In the case of a fiber-optic modal-domain sensor, it is valuable to use a "source lead fiber" to couple the light source to the sensing fiber, or to use a "detection lead fiber" to couple the sensing fiber to the detector, or both. In a fiber sensor system with one or both lead fibers, the system should be highly sensitive to disturbances in the sensing fiber and insensitive to disturbances in the lead fiber or fibers.
Several prior art references have discussed means for avoiding signal distortion due to disturbances which do not impact the sensor fiber. Jeunhomme U.S. Pat. No. 4,843,233 describes an insensitive source lead system with a singlemode fiber 2 disposed between the coherent light source 1 and the multimode sensor fiber 3. In addition, one or more optical fibers 5n with core diameters smaller than that of the multimode sensor fiber 3 are placed between the multimode sensor fiber and the detectors 6.sub.n in order to provide restriction. This restriction means can have an economic and/or performance disadvantage, due to the combination of fibers with different core diameters. Also, optimizing the fiber parameters for system performance may require a special fiber that is not mass produced for other applications and is therefore expensive.
Martin, Le Boudec and Jeunhomme, "Integrating Fiber Optic Vibration Sensor", SPIE Vol. 985 Fiber Optic and Laser Sensors VI (1988), pp. 344-348, also describe a small core returning fiber, as well as the use of laterally offset sensor and return fibers in order to convert "the spatial modulation of the speckle pattern into [the] intensity modulation of the detected light". (page 345, FIG. 2). This system has the serious disadvantage that a special component is needed to establish and maintain the offset, which may be costly because most commercially available fiber-optic components are coaxial and do not create controlled lateral offsets.
Butter U.S. Pat. No. 4,297,684 describes a method of de-sensitizing sections of a sensor fiber by shielding them within a length of copper tubing (col. 2, 11. 34-40). Leung et al., "Fiber optic line-sensing system for perimeter protection against intrusion", Tech. Digest, OFS 86, Tokyo, Japan, p. 113, describes burying insensitive sections at a deeper level than sensitive sections. This kind of protection for sections of a sensor fiber is impractical for many applications, and expensive even where practical.